An electronic amplifier is a device for increasing the power of a signal. An electronic amplifier increases the power of a signal by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier may be considered as modulating the output of the power supply. Amplifiers can be specified according to their input and output properties. Amplifiers can have some kind of gain, or multiplication factor relating the magnitude of the output signal relative to the input signal. The gain may be specified as the ratio of output voltage to input voltage (voltage gain), output power to input power (power gain), or some combination of current, voltage and power. The power gain of an amplifier depends on the source and load impedances used as well as its voltage gain; while an RF amplifier may have its impedances optimized for power transfer, audio and instrumentation amplifiers are normally employed with amplifier input and output impedances optimized for least loading and highest quality.
Different amplifier designs can be referred to as a class of amplifier. In a class A amplifier, an input signal is amplified over the input signal's entire cycle. Class A amplifiers are relatively inefficient, and can have an efficiency lower than 50%. In a class B amplifier, an input signal is amplified over half the input signals entire cycle. A class B amplifier is relatively efficient up to about 78.5% efficient, adds a relatively large amount of distortion. Class-C amplifiers can conduct less than 50% of the input signal and the distortion at the output is high, but high efficiencies (up to about 90%) are possible.